fdo#72488: Broken text when showing visible space

Turning on showing nonprinting characters replaces the space with bullet
character, but still draws the text with the original kern array, this
works fine until there are ligatures involving the space character as
the number of glyphs after replacing the space with the bullet will be
different and the kern array will be completely off.

This is a hack that gives up on replacing the space with a bullet when
its width is zero, not sure if it would interfere with other legitimate
uses.

Change-Id: If5a929dd6acffe018dc26f15ba31c9f91294d256
Reviewed-on: https://gerrit.libreoffice.org/7004
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>